Thank you Rich,

“so if you have some PAM module that can auth against AD (except LDAP which probably won't work) you can configure PAM passthrough to pass the auth to that PAM module, then to AD”

Are you implying, the FDS will go out of picture with PAM? I mean, can I still use FDS to check the uid attribute and then pass it to PAM?
I am sorry, but I am not getting the flow clearly.

Can you type in rough, how the flow goes? (Hopefully someone might come this way and find this helpful)